How Vaccines Work: Training Your Immune System

A vaccine is a way of training your immune system before you meet a dangerous germ. It works by imitating an infection without causing the disease.

Different vaccines do this in different ways. Some contain a weakened or inactivated (killed) form of the germ. Others contain just one part of it, such as a surface protein. Newer vaccines carry the genetic instructions (DNA or RNA) that let your own cells make that harmless part for a short time.

The part of a germ that the immune system learns to recognise is called an antigen. Every vaccine is built around one or more antigens.

After a jab or oral drops, the antigen is picked up by antigen-presenting cells, such as dendritic cells and macrophages. These cells swallow the antigen, break it up, and display pieces of it on their surface.

They then show these pieces to T cells, a type of white blood cell. This is how the immune system learns that something new has arrived and which shape it needs to fight.

Once alerted, helper T cells switch on the B cells whose receptors fit the antigen. Those B cells multiply, and many of them become plasma cells that release antibodies: Y-shaped proteins that lock onto one specific antigen, a bit like a key that fits only one lock.

This first response is slow. It takes days or even weeks to build up, which is why a vaccine does not protect you immediately and why you can still catch an infection in the weeks straight after vaccination.

Some of the B and T cells do not join the first fight. Instead they become long-lived memory cells that remember the antigen.

If the real germ enters your body later, these memory cells recognise it straight away. The second response is much faster and much bigger: memory B cells can produce tens to hundreds of times more antibody than in the first response. The germ is usually cleared before it can make you seriously ill. Many vaccines need two or more doses, given weeks or months apart, to build strong, lasting memory.

In 1994, India accounted for around 60 per cent of the world's polio cases. Through the Pulse Polio programme, health workers set out to give polio drops to every child, whether at home, at school or while travelling. On a single national immunisation day, around 1.2 million vaccinators took part.

The effort worked. India recorded its last polio case on 13 January 2011, in Howrah, West Bengal, and the World Health Organization certified the country polio-free on 27 March 2014. India has since added an injectable inactivated polio vaccine to its routine immunisation programme.

When enough people in a community are vaccinated, a germ struggles to spread because most of the people it meets are immune. This is called herd immunity. It protects people who cannot be vaccinated, such as newborn babies or those with certain illnesses, which is why every vaccinated person helps keep others safe.